Heretofore, as the constituent element members of semiconductor devices, photosensitive devices for use in electrophotography, image input line sensors, image pickup devices or other optical devices, there have been proposed a number of amorphous semiconductor films, for instance, an amorphous deposited film composed of an amorphous silicon material compensated with hydrogen atoms (H) or/and halogen atoms (X) such as fluorine atoms or chlorine atoms (hereinafter referred to as "a-Si(H,X)"). Some of such films have been put into practical use.
Along with those amorphous semiconductor films, there have been proposed various methods for their preparation using plasma chemical vapor deposition technique wherein a raw material gas is decomposed by subjecting it to the action of an energy of direct current, high frequency or microwave glow discharge thereby to form a deposited film on a substrate of glass, quartz, heat-resistant resin, stainless steel or aluminum. And there have also been proposed various apparatus for practicing such methods.
Recently, public attention has been focused on the plasma chemical vapor deposition method by means of microwave glow discharge decomposition (hereinafter abbreviated as "MW-PCVD method") at the industrial level.
One representative apparatus for practicing such MWPCVD method is such that has a construction as shown in a schematic perspective view of FIG. 2.
The MW-PCVD apparatus shown in FIG. 2 comprises a substantially enclosed reaction chamber 201 (film-forming chamber) having a microwave introducing window 201 which is hermetically provided with the reaction chamber. The microwave introducing window 202 is formed of a dielectric material such as quartz, glass, alumina ceramics, or the like. Numeral reference 203 stands for a metal waveguide being connected to the microwave introducing window 202. The waveguide 203 is extending from a microwave power source (not shown) through a matching box and an isolator (not shown). The reaction chamber 201 is provided with a gas feed pipe 208 and an exhaust pipe 204 having one end opened into the reaction chamber 201 and the other end communicated with an exhaust device (not shown). The reaction chamber 201 has a discharge space formed by circumferential walls thereof. Numeral reference 307 stands for a substrate holder having a substrate 305, on which a film is to be deposited, being placed thereon.
In this conventional apparatus, a deposited film is formed in the manner as follows. The reaction chamber 201 is evacuated by the action of the exhaust device to thereby bring the inner pressure of the reaction chamber to a level of lower than 1.times.10.sup.-7 Torr. Then, an electric heater (not shown) built in the substrate holder 205 is activated to heat the substrate 205 to a temperature suitable for forming a deposited film thereon and the substrate is maintained at this temperature. And in the case of forming an amorphous silicon deposited film, for example, a raw material gas such as silane gas (SiH.sub.4) is introduced into the reaction chamber 201 through the gas feed pipe 208 at a predetermined flow rate. Concurrently, the microwave power source is switched on to generate microwave having a frequency of higher than 500 MHz, preferably, of 2.45 GHz, followed by introducing the microwave energy into the reaction chamber 201 through the waveguide 203 and the microwave introducing window 202, whereupon the raw material gas is excited with the action of the microwave energy and dissociated to cause the formation of a deposited film on the surface of the substrate 205 in the discharge space 206 of the reaction chamber 201.
The apparatus shown in FIG. 2 makes it possible to form a deposited film as described as long as the substrate is of flat or almost flat shape. However, it is difficult for the apparatus to form a deposited film, for example, on a cylindrical substrate for a photosensitive member to be used in electrophotography.
There has been proposed an apparatus for forming a deposited film on a cylindrical substrate, as shown in FIG. 3, which is designed to manufacture an electrophotographic photosensitive drum.
More specifically, the film-forming apparatus shown in FIG. 3 has a substantially enclosed reaction chamber (film-forming chamber) 301 comprising a circumferential wall having an end portion thereof hermetically provided with a microwave introducing window 302 to which a waveguide 303 extending from a microwave power source (not shown) is connected. The reaction chamber 301 has a plasma generation space (discharge space in other words) 306 circumscribed by a plurality of rotatable cylindrical substrate holders each having a cylindrical substrate 305 being positioned thereon. Each of the substrate holders is provided with an electric heater 307 for adjusting the temperature of the substrate 305 positioned thereon. The reaction chamber 301 is provided with an exhaust means 304 being connected through an exhaust valve (not shown) to an exhaust device (not shown). Numeral reference 308 stands for gas feed means which serves to supply a raw material gas into the discharge space 306. The gas feed means 308 is provided behind each pair of cylindrical substrates 305 so as to supply a raw material gas through the space between the two cylindrical substrates 305 into the discharge space 306. Each of the gas feed means 308 is connected through a conduit 308' to a reservoir (not shown).
The film-forming apparatus shown in FIG. 3 operates substantially in the same manner as the apparatus shown in FIG. 2, except that the cylindrical substrates are rotated during the film-forming process in order to form a desired deposited film on each of the cylindrical substrates 305.
The film-forming apparatus shown in FIG. 3 makes it possible to mass-produce photosensitive drums having practical applicable electrophotographic properties which can be used in electrophotographic image-forming process. But those photosensitive drums prepared by the film-forming apparatus shown in FIG. 3 are apt to cause various problems upon use in the image-forming process in that images as copied become undesirably thin in density, accompanied with fogginesses, poor in incontrast or the like, particularly when the electrophotographic image-forming process is carried out under high humidity charging or low humidity developing, or when there are differences in the functions of developers used, or when these situations take place together.
In addition to the above, there are problems for the film-forming apparatus shown in FIG. 3 because the gas feed means 308 situated behind and close to the cylindrical substrates 305, dusts or foreign matters present on or around the gas feed means are blown up to get in the discharge space 306 upon supplying a raw material gas, whereby those dusts or foreign matters are contaminated into the deposited films formed on the cylindrical substrates 305. If this happens, the resultant photosensitive drums become uneven in the electrophotographic properties.